How Do You Get a Sinus Infection? Causes and Triggers

Most sinus infections begin with an ordinary cold. When a virus inflames the lining of your sinuses, the narrow drainage passages swell shut, mucus pools, and conditions become ideal for infection to take hold. Roughly two out of five people with a common cold show sinus involvement by the first week of illness, yet only a small fraction of those cases progress to a bacterial infection requiring treatment. The story of how you get a sinus infection is really a story about blocked drainage, and the surprisingly long list of things that can cause it.

It Usually Starts With a Virus

The single most common route to a sinus infection is a viral upper respiratory infection. Rhinoviruses, influenza, and parainfluenza viruses are the usual culprits. In a study tracking patients through an ordinary cold, about 39% had radiographic evidence of sinusitis by day seven, and viral infection was detected in over 80% of those sinusitis cases.¹ That means most “sinus infections” are really viral sinusitis, not bacterial, and they resolve on their own as the cold clears.

The mechanism is straightforward. Your sinuses are air-filled cavities behind your forehead, cheeks, and eyes, all lined with mucus-producing tissue. Each sinus drains through a small opening into the nasal passages. When a virus infects that lining, it swells, the opening narrows or closes, and mucus that would normally drain gets trapped. The virus also directly damages the cilia, the tiny hair-like structures responsible for sweeping mucus out of your sinuses. Research shows that influenza, for example, reduces the expression of proteins needed for cilia to beat properly, slowing mucociliary clearance and weakening one of the body’s primary defenses against infection.² Multiple types of pathogenic organisms, including viruses, bacteria, and fungi, specifically target cilia and epithelial cells in the airways, damaging the barrier that keeps infections from gaining a foothold.³

By day 21 of a cold, sinus involvement typically drops back down to around 11%, which tells you that most viral sinusitis clears without any specific treatment.¹ The trouble starts when it doesn’t.

When Bacteria Move In

A bacterial sinus infection is typically a secondary event. It happens when a virus or allergic reaction has already blocked the sinuses, creating a warm, stagnant environment where bacteria can thrive. Most sinus infections are viral, and only a small proportion develop a secondary bacterial infection.⁴ The clinical rule of thumb is that if your symptoms are worsening after about ten days, or if they initially improved after a cold and then get worse again (“double worsening”), bacteria have likely become involved.

The bacteria most commonly responsible for acute bacterial sinusitis in both adults and children are Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis.⁴ A large retrospective study of pediatric sinus infections found pathogens in over 92% of samples, with Moraxella catarrhalis as the most frequently isolated organism, followed by H. influenzae and S. pneumoniae.⁵ In chronic sinusitis, the cast of characters shifts: Staphylococcus aureus and anaerobic bacteria become the dominant players.⁴

This distinction matters because antibiotics are useless against viral sinusitis, which accounts for the vast majority of cases. Taking antibiotics for a sinus infection that hasn’t become bacterial won’t speed your recovery and contributes to antibiotic resistance. This is why doctors often recommend watchful waiting for the first week to ten days of symptoms.

Allergies Set the Stage

If you have allergic rhinitis, you’re at substantially higher risk of developing sinus infections. Allergies cause the nasal lining to swell and produce excess mucus through the same inflammatory pathways that viruses exploit. The result is the same: blocked sinus drainage, mucus buildup, and an inviting environment for infection. A cross-sectional analysis using national health survey data found that people with allergy symptoms had a sinusitis rate of about 22%, compared to roughly 7% in those without allergies, translating to nearly a fourfold increase in risk.⁶

The problem is compounded because allergic inflammation doesn’t just cause congestion; it degrades the barrier function of your respiratory lining, making it easier for infections to take hold. People with allergic rhinitis who develop acute bacterial sinusitis tend to have a more severe course of the illness.⁷ Managing underlying allergies, whether through avoidance, antihistamines, or nasal corticosteroid sprays, is one of the more effective ways to reduce your frequency of sinus infections.

Air pollution is another environmental contributor. Particulate matter and other airborne pollutants are increasingly recognized as factors that disrupt the epithelial barrier in the sinuses and promote chronic sinonasal inflammation.⁸ If you live in a high-pollution area or work around dust, fumes, or chemical irritants, your sinuses are taking regular low-grade hits that make infection more likely over time.

Dental Problems That Reach Your Sinuses

Here’s a cause many people never consider: your teeth. The roots of your upper premolars and molars sit remarkably close to the floor of your maxillary sinuses, sometimes separated by just a thin sheet of bone. When those teeth develop infections, undergo extractions, or suffer trauma, the infection can spread directly upward into the sinus cavity.⁹

This condition, called odontogenic sinusitis, accounts for a meaningful share of maxillary sinusitis cases and is frequently missed on initial evaluation. The most common non-iatrogenic cause is apical periodontitis, an infection at the tip of a tooth root, and the teeth most commonly involved are the premolars and molars because of their proximity to the sinuses.¹⁰ Dental procedures like extractions, implant placements, and root canals can also introduce bacteria or create openings between the mouth and the sinus.

A clue that your sinus infection might be dental in origin is that it only affects one side. Most viral and allergic sinusitis tends to be bilateral. A foul-smelling or foul-tasting nasal discharge is another red flag. If you’ve had recurring sinus infections on one side, particularly after dental work, it’s worth raising the dental connection with your doctor.

Anatomy That Works Against You

Some people are structurally predisposed to sinus infections. A deviated nasal septum, where the wall between your nasal passages is shifted to one side, is one of the most common anatomical variations. It’s extremely prevalent: in one study of patients with chronic maxillary sinusitis, about 78% had some degree of septal deviation.¹¹ The deviation alters airflow and drainage, reducing sinus volume on the deviated side and increasing mucosal thickening, which showed up in over 84% of patients and was concentrated on the deviated side.¹¹

That said, having a deviated septum doesn’t automatically sentence you to chronic sinus problems. Reviews of the evidence suggest that only severe deviations contribute meaningfully to the development of chronic rhinosinusitis, and milder deviations may not raise your risk at all.¹² Other anatomical variants that can compromise sinus drainage include concha bullosa (an air-filled enlarged turbinate bone), prominent agger nasi cells, and bony spurs in the septum.¹² All of these can narrow the osteomeatal complex, the critical drainage pathway where the sinuses empty into the nose.

Fungal Sinusitis

Fungi are an uncommon but clinically important cause of sinus infections. Fungal sinusitis falls into two broad categories: invasive and noninvasive. The invasive forms, where fungal tissue actually penetrates the sinus lining and sometimes the bone, are rare and occur almost exclusively in people with weakened immune systems, such as those undergoing chemotherapy, organ transplant recipients, or people with uncontrolled diabetes.¹³ Acute invasive fungal sinusitis is a medical emergency with a high mortality rate if not treated aggressively.

The noninvasive forms are more common and far less dangerous. A fungus ball, sometimes called a mycetoma, is a clump of fungal debris that grows inside a sinus cavity without invading the tissue. Allergic fungal sinusitis is an immune-mediated reaction to fungal material in the sinuses, often seen in younger patients with a history of allergies or asthma. These subtypes have different clinical features, treatments, and outcomes.¹⁴ If you have a healthy immune system, invasive fungal sinusitis is something you’re very unlikely to encounter, but allergic fungal sinusitis can be a persistent problem for people with strong allergic tendencies.

Immune Deficiencies and Genetic Conditions

When someone gets sinus infections far more often than seems reasonable, or when infections keep coming back despite proper treatment and even surgery, an underlying immune or genetic condition may be at work. Immune deficiencies are more common in people with chronic rhinosinusitis than in the general population and should be especially considered in patients who don’t respond to standard medical and surgical therapy.¹⁵

Common variable immunodeficiency and selective IgA deficiency, conditions where the body doesn’t produce enough of certain antibodies, carry a high prevalence of chronic sinusitis.¹⁶ Cystic fibrosis and primary ciliary dyskinesia are inherited disorders that directly impair mucociliary clearance, the cilia-driven system that sweeps mucus out of the sinuses.¹⁷ Both conditions lead to thick, stagnant secretions and chronic sinus inflammation that begins in childhood and persists throughout life. The chronic rhinosinusitis seen in both conditions shares similar tissue-level characteristics despite looking somewhat different on endoscopy.¹⁸

These conditions are rare, but recognizing them is important because the treatment approach changes significantly. Chronic sinusitis driven by an immune deficiency may require immunoglobulin replacement therapy rather than yet another round of antibiotics or a repeat surgery.

Biofilms and Why Some Sinus Infections Never Fully Clear

If you’ve ever wondered why chronic sinusitis can be so maddeningly persistent, even after surgery, bacterial biofilms are a major part of the answer. A biofilm is a structured community of bacteria encased in a self-produced protective matrix. Unlike free-floating bacteria, biofilm bacteria are highly resistant to antibiotics and to your immune system’s attempts to clear them.

Biofilm formation in the sinuses is associated with worse symptoms, worse imaging findings before surgery, and worse outcomes after surgery. Patients with biofilms on their sinus tissue are more likely to have ongoing symptoms after surgical intervention, require extra post-operative visits, and need multiple courses of antibiotics.¹⁹ Staphylococcus aureus and Pseudomonas aeruginosa biofilms are particularly associated with poor clinical improvement.¹⁹ The condition is widespread enough that biofilm formation is now considered a key driver of the refractory nature of chronic rhinosinusitis.²⁰

Research into the sinus microbiome adds further nuance. Chronic rhinosinusitis appears to be associated with reduced microbial diversity in the sinuses, though whether this reduced diversity causes the disease or results from it remains an open question.²¹ Sequencing studies consistently find an enrichment of potentially harmful species like S. aureus and the presence of biofilm-associated community states in sinuses affected by chronic inflammation.²² It’s a chicken-and-egg problem that researchers are still working to untangle, but the practical upshot is that restoring a healthy microbial community in the sinuses may eventually become a treatment strategy.

Hospital-Acquired Sinusitis

Sinus infections don’t always come from colds or allergies. In hospital settings, particularly in intensive care, sinusitis is a recognized complication of nasogastric tubes and prolonged intubation. A tube running through the nose physically obstructs sinus drainage in much the same way that swollen tissue does, and the risk is significant: one study found that the presence of nasogastric tubes was linked to a dramatically increased odds of sinusitis in mechanically ventilated patients.²³ Nosocomial sinusitis can be a hidden source of fever in ICU patients and may contribute to sepsis if unrecognized.²⁴

This isn’t something most people need to worry about in everyday life, but it’s worth knowing about if you or a family member faces a prolonged hospital stay involving nasal tubes. Medical teams in ICUs are generally aware of this risk and monitor for it, but if an unexplained fever develops in someone with a nasogastric tube, sinusitis should be on the list of possible explanations.

The “Sinus Headache” That Is Actually a Migraine

One of the most common misconceptions about sinus infections isn’t about what causes them, but about what counts as one. Millions of people believe they have recurring sinus headaches and treat themselves accordingly, often with decongestants and sometimes with unnecessary antibiotics. In reality, a large proportion of self-diagnosed sinus headaches are migraines. A meta-analysis of the available research found that among patients initially diagnosed with sinus headache, the pooled prevalence of actual migraine was about 55%, climbing to roughly 65% when tension-type headaches were also included.²⁵

The confusion arises because migraines can cause facial pressure, nasal congestion, and even a runny nose, all of which feel like sinus symptoms. In studies of patients with confirmed migraine who had previously been told they had sinus headaches, misdiagnosis rates reached as high as 81%, with some patients going decades before getting a correct diagnosis.²⁵ If you get recurring “sinus headaches” without the classic signs of infection (thick, discolored discharge, fever, or symptoms that clearly follow a cold), it’s worth exploring whether migraine might be the actual culprit. Migraine-specific treatments work far better than decongestants for people in this group.

Pregnancy and Hormonal Shifts

Pregnancy rhinitis is a nasal condition that affects a notable percentage of pregnant women and often goes undiagnosed or gets mistaken for an allergy or infection. It’s driven by hormonal changes, particularly rising estrogen and progesterone along with placental growth factors, that cause the nasal lining to swell and produce more mucus.²⁶ While pregnancy rhinitis itself is not an infection, the resulting chronic nasal congestion and impaired drainage create the same conditions that predispose anyone to sinusitis. Pregnant women dealing with persistent stuffiness should be aware that their risk of developing an actual sinus infection is elevated, and that treatment options are more limited during pregnancy. Saline irrigation is generally considered safe and can help keep the sinuses draining, while many standard medications require medical guidance during pregnancy.

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